1. Field of the Invention
This invention relates to gain control circuits and particularly to an improved gain control circuit in which the dynamic range thereof is independent of the control voltage applied thereto.
2. The Prior Art
Many types of gain control circuits have been proposed, each of which has some advantages. When a circuit having certain advantages is installed in an appropriate system, it may be required to operate under circumstances for which it is not particularly suited.
One type of gain control circuit proposed heretofore, includes an input transistor, the emitter of which is connected to the base of one of the transistors of a differential amplifier. The common circuit point between the emitter of the input transistor and the base of the transistor to which it is connected is further connected by a diode to a first controllable current source. The common emitter circuit of the transistors of the differential amplifier is connected to a second controllable current source, and the gain of this type of control circuit is approximately determined by the ratio of current in the second current source to the current in the first current source.
That type of gain control circuit also has some disadvantages. When the current of the first current source is reduced to a small value to make the gain of the circuit large, the output impedance of the input transistor increases. As a result, the gain of the circuit is changed more than is expected.